Process of making niobium stannide bodies



United States Patent 3,270,400 PROCESS OF MAKING NIOBIUM STANNIDEBODIES, Eugen J. Saur, 20 Jahnstn, Giessen, Germany N0 Drawing. FiledAug. 9, 1963, Ser. No. 301,167 3 Claims. (Cl. 29155.5)

This application is a continuation-in-part of my copending application,Serial No. 208,925, file-d July 10, 1962 and Serial No. 233,961, filedOctober 29, 1962.

The present invention relates to superconducting electromagnets e.g.solenoids, particularly of the type wherein a thin coating of Nb Sn isformed on a refractory metal wire which is then wound into a solenoid.

Ductile forms of such wires have been made as described in the copendingapplication of Allen and Stauffer, S.N. 102,593, filed April 12, 1961and my own copending applications S.N. 208,925, filed July 10, 1962 andSN. 233,961, filed October 29, 1962. While these wires usually retaintheir critical current capacities after being wound into a solenoid,they can experience a lowering of critical current due to excessivestresses induced by mishandling. It appears that the reason for thislowering of critical current is that superconducting filaments withinthe Nb Sn coating are broken. These filaments can be restored orreplaced through annealing of the wire. It would be expected that theannealing would have to take place at about 9001200 C. since these arethe temperatures at which Nb Sn is formed in the course of making thesuperconductive wire. This would be prohibitive since many intermediatetemperature insulations, which may be applied to the wire, cannotwithstand such temperatures. As used herein, low temperature insulationsrefers to plastics which can withstand less than about 100 C.;intermediate temperature insulations refers to Teflon, silicone rubber,magnesium fluoride which can withstand temperatures between 100 and insome cases up to about 800 C.; and high temperature insulations refersto ceramic insulation which can withstand 1000 C.

It is therefore the object of the invention to provide a method ofcompensating for the effects of excessive stresses and to restore thecritical currents of damaged superconductive wires and a further objectthat such a method will be compatible with conventional, intermediatetemperature dielectric insulations which are applied to the wire.

In accord with the present invention, Nb Sn wires may be wrapped intotight solenoids. Where the stresses of such winding cause the criticalcurrent to decrease, the original high critical current values may berestored by annealing at ZOO-600 C. I have discovered that heating atthese temperatures is sufficient to restore high critical currents.

In a preferred embodiment of the invention, superconductive wire isprepared by treating a niobium surface to render it wettable by tin,applying a coating of tin and heating the coated wire at 1100 C. to forma diffusion coating of Nb Sn. The details of the above process are setforth more fully in my above copending application of which the presentapplication is a continuationin-part. The wires made according to mymethod are characterized in the presence of substantial amounts ofunreacted tin over the Nb Sn coating.

The superconductive wire is then coated with copper and with an outerlayer of a temperature-resistant dielectric such as Teflon or magnesiumfluoride. The resultant product is then wound into a solenoid andtested. When the testing reveals that the critical current readings arelower than those to be expected, the solenoid can be placed in an argonfurnace and heated to the upper tin in the Nb Sn coatings.

"ice

temperature limit of the dielectric. This heating will restoresuperconducting paths which may have been breached by stressesencountered in winding the solenoid. All the materials used in thesolenoid are capable of withstanding this temperature range. It will beappreciated that, in the absence of the present method, it would benecessary to scrap faulty solenoids.

Some non-limiting examples showing the ability of the present method torestore the critical current of Nb Sn wires are now set forth.

Example 1 A niobium wire was cleaned by heating at 1100 C. in vacuum forone hour over a molten tin bath. The wire was then dipped into the tinbath for 4 minutes and postheated for 15 minutes. The apparatus is thesame as that shown in my copending application, S.N. 233,961, filedOctober 29, 1962. The resultant coated wire, shown in FIG. 1, wasductile and had a critical current of 20 amps in a transverse magneticfield of 30 kilogauss, at 4.2 K. This indicates that substantial Nb Snpaths were present in the outer coating of the wire.

The wire was then strained by bending it about a spool having a 60millimeter radius. After this distortion the wire was tested forcritical current and had 15% less, only 17 amps at 30 kilogauss and 4.2K.

Then the wire was heated in vacuum at 250 C. for 12 hours and retested.The critical current was then 20 amps at 30 kilogauss and 4.2 K.

Example 2 The process of Example 1 was repeated save that in this casethe substrate was a niobium ribbon, rather than a round wire. Thecross-sectional dimensions of the ribbon were .05 millimeter thick and 2millimeters wide. The Nb Sn coated ribbon had a critical current of 40amps at 30 kilogauss and 4.2 K. Then the ribbon/was bent about a spoolhaving a 10 millimeter radius. After this distortion, it had a criticalcurrent of 20 amps at 30 kilogauss and 4.2 K.

The ribbon was heat treated at 250 C. for 12 hours. After thistreatment, it had a critical current of 28 amps at 30 kilogauss and 4.2K.

Example 3 The process of Example 2 was repeated for a niobium ribbonwhich was .2 millimeter thick by 2.4 millimeters wide. Beforedistortion, the critical current was 50 amps. After bending about a 10"millimeter radius, the critical current was 25 amps. Heating the ribbonat 250 C. for 12 hours restored its critical current to 35 amps. Allcritical current tests were at 30 kilogauss and 4.2 K.

Example 4 The tests of Examples 2 and 3 were repeated with heattreatments after distortion at C. for 12 hours. No improvement wasobtained. The critical current of the .05 by 2 ribbon remained at 20amps and the critical current of the .2 by 2.4 ribbon remained at 25amps.

It is necessary to make the wires or ribbons with excess It is believedthat this facilitates low temperature healing by permitting brokensections of superconductive filaments to move easily towards each otherand to be rejoined. It is preferred that the healing heat treatment becarried out in a manner to avoid reacting all the excess tin.

Substantial healing can be obtained by heating the damaged wire orribbon above 250 C. for times in excess of 10 hours. At highertemperatures, the times can be reduced. In any event, care should betaken to avoid completely reacting all the tin in the wire or ribbonwhen higher temperatures are used.

What is claimed is:

1. A method of making electromagnets, such as solenoids and the like,comprising the steps of preparing an elongated superconductor with athin layer of Nb Sn and excess tin on a refractory metal base,electrically insulating the superconductor with an insulation capable ofwithstanding temperatures in the range of from at least 200 C. to about800 C., winding the superconductor into a solenoid, testing the solenoidto detect :any loss of critical current in the wire, then heating thesolenoid between 200 C. and the upper temperature limit of theinsulation whenever a lowering of critical current capacity is indicatedby the test for a time sufficient to restore at least a portion of thelost critical current capacity but less than the period required toconsume all of the excess tin.

2. The method of claim 1 wherein the superconductor is coated with aninner sheath of copper and an outer sheath of the insulation prior towinding into a solenoid.

3. A method of making superconductive elements comprising the steps ofpreparing adjacent layers of niobium 4 and tin, heating the layers at9001200 C. to form a diffusion layer of Nb Sn and excess tin, testingthe resultant product for critical current, working the resultantproduct into a desired shape and retesting for critical current, andheating the superconductor between 200 C. and 600 C. whenever loweringof the critical current capacity is indicated, the heating beingcontinued for a sufi'lcient time and sufficiently low temperature torestore a substantial portion of the critical current without reactingall the excess tin.

References Cited by the Examiner UNITED STATES PATENTS 3,163,832 12/1964Nahrnan et al. 29-155.5 3,181,936 5/1965 Perry et al. 29155.5 3,218,69311/1965 Allen et al. 29155.5

JOHN F. CAMPBELL, Primary Examiner.

P. M. COHEN, Assistant Examiner.

1. A METHOD OF MAKING ELECTROMAGNETS, SUCH AS SOLENOIDS AND THE LIKE,COMPRISING THE STEPS OF PREPARING AN ELONGATED SUPERCONDUCTOR WITH ATHIN LAYER OF NB3SN AND EXCESS TIN ON A REFRACTORY METAL BASE,ELECTRICALLY INSULATING THE SUPERCONDUCTOR WITH AN INSULATION CAPABLE OFWITHSTANDING TEMPERATURES IN THE RANGE OF FROM AT LEAST 200*C. TO ABOUT800*C., WINDING THE SUPERCONDUCTOR INTO A SOLENOID, TESTING THE SOLENOIDTO DETECT ANY LOSS OF CRITICAL CURRENT IN THE WIRE, THEN HEATING THESOLENOID BETWEEN 200*C. AND THE UPPER TEMPERATURE LIMIT OF THEINSULATION WHENEVER A LOWERING OF CRITICAL CURRENT CAPACITY IS INDICATEDBY THE TEST FOR A TIME SUFFICIENT TO RESTORE AT LEAST A PORTION OF THELOST CRITICAL CURRENT CAPACITY BUT LESS THAN THE PERIOD REQUIRED TOCONSUME ALL OF THE EXCESS TIN.
 3. A METHOD OF MAKING SUPERCONDUCTIVEELEMENTS COMPRISING THE STEPS OF PREPARING ADJACENT LAYERS OF NIOBIUMAND TIN, HEATING THE LAYERS AT 900-1200*C. TO FORM A DIFFUSION LAYER OFNB3SN AND EXCESS TIN, TESTING THE RESULTANT PRODUCT FOR CRITICALCURRENT, WORKING THE RESULTANT PRODUCT INTO A DESIRED SHAPE ANDRETESTING FOR CRITICAL CURRENT, AND HEATING THE SUPERCONDUCTOR BETWEEN200* C. AND 600*C. WHENEVER LOWERING OF THE CRITICAL CURRENT CAPACITY ISINDICATED, THE HEATING BEING CONTINUED FOR A SUFFICIENT TIME ANDSUFFICIENTLY LOW TEMPERATURE TO RESTORE A SUBSTANTIAL PORTION OF THECRITICAL CURRENT WITHOUT REACTING ALL THE EXCESS TIN.